In ink jet printers and in particular in those which receive the data to be printed from separate apparatuses, such as for example a central processing and printing unit for data, or a facsimile apparatus, it is preferable to prevent a situation in which the ink contained in the reservoir of the printing head becomes exhausted unforeseeably, suddenly interrupting any printing operation which is in progress.
Various prior art, methods and devices are known for overcoming the aforementioned disadvantage.
In U.S. Pat. No. 4,202,267 there is described a system for indicating the expiry of the ink in the reservoir of an ink jet printing head, by means of a pair of electrodes on the floor of the ink reservoir and wetted by the ink. The electrodes are connected to a detection circuit to indicate the variations of the electrical resistance of the ink contained between the electrodes. When the ink is close to expiry one of the electrodes becomes uncovered and the detection circuit reveals a very high resistance thereby indicating expiry of the ink. Such a system provides no advance indication of the expiry of the ink as it only indicates when the reservoir is virtually empty. Also it may give false alarms when the sloshing movements caused by the to and fro movement of the head results in one of the electrodes being temporarily uncovered by the ink. Moreover, the presence of the electrodes and of the associated electrical connections complicates the construction of the reservoir and makes the latter more costly.
The European patent application No. 509747 discloses another device for the detection of the ink in the reservoir of an ink jet printing head. In this a pair of electrodes is introduced into the reservoir, which is filled with porous material soaked in ink, in a zone in which the capillarity of the porous material is greater than in the rest of the reservoir. As a result of this, the zone is emptied last as ink is used. In this arrangement the electrodes detect a more rapid increase in the electrical resistance of the ink, thus indicating in advance the impending expiry of the ink.
Such a device, although eliminating some of the disadvantages noted in the system of the aforementioned U.S. patent, such as those caused by the sloshing movement of the ink, proves to be of complex and costly construction as a result of the presence of the electrodes and of the associated electrical contacts.